The present invention relates to a solar heat collection element (HCE), which is a tubular radiation absorbing device for solar thermal applications.
HCEs may be employed in, among other applications, a parabolic trough-collector for a solar power plant. With reference to FIG. 1, the conventional heat collection structure comprises a plurality of HCEs 10, a pair 10A, 10B of which are shown coupled together. Each HCE 10 includes a central tube 12 and a glass tubular jacket (outer tube) 14 surrounding the central tube 12 so as to form a cylindrical space (with a ring-shaped cross section) 16 therebetween. Solar radiation is concentrated via a parabolic tracking mirror (not shown) and focused on the HCE 10 and converted into heat. The conventional HCE 10 is about four meters long, and the overall length of the heat collection structure is based on the number of HCEs coupled together. The collected heat is conducted away via a heat-carrying medium flowing through the central tube 12 of the HCE 10 and is used directly as process heat or converted into electrical energy.
The central (or inner) tube 12 is typically stainless steel and coated with an interference coating designed to: (1) absorb solar radiation, (2) act as an infrared mirror, and (3) have low emissivity at a maximum temperature of operation. The outer tube 14 is typically formed from a borosilicate glass. The inner steel tube 12 and the outer glass tube 14 are connected using bellows 18 to compensate for the thermal expansion mismatch between glass and steel. The outer glass tube 14 is bonded to the bellows 18 using glass-to-metal sealing techniques and the bellows 18 is welded to the steel tube 12. The bellows 18 provides a glass-metal transitional element, which permits longitudinal movement between the steel and glass tubes 12, 14 to compensate for the thermal expansion mismatch therebetween.
The space between the glass tube 14 and the steel tube 12 is evacuated to about 10−4 Torr to minimize radiating heat losses. Conventional HCE design employs organic heat transfer fluid (HTF) through the steel tube 12. Typical HTFs include Therminol™ VP-1 or Dowtherm A, each a mixture of about 75% diphenyl oxide and 25% biphenyl.
The operating temperature range of the conventional HCE 10 that uses synthetic oil as the HTF is between about 300-400° C. (750° F. maximum) for solar power generation. At the maximum operating temperature of 400° C., the glass tube 14 heats up to about 100° C. To meet quality control and reliability requirements for tubes using synthetic oil as the HTF, the HCEs must be capable of cycling from below 0° C. to 400° C. for a period of 25-30 years.
Free hydrogen generated by age degradation of the synthetic oil HTF diffuses through the steel tube 12 and compromises the vacuum within the cylindrical space 16. The permeation rate and the oil degradation rate increase with increasing operating temperature. Hydrogen gas has very high thermal conductivity so there is a significant heat loss associated with leakage of hydrogen into the space 16 and resultant reduction of vacuum.
The use of supersaturated steam and molten salts as HTFs are being considered to mitigate the hydrogen diffusion problem. The concern with using supersaturated steam as an HTF is that the weight of the steel tube 12 would need to increase due to the higher pressures required—compared with the pressures used with synthetic oil. Further, a design using supersaturated steam must also deal with flash steam, condensation, safety precautions (i.e., tube breakage in the field), etc. The major concerns with the use of molten salts as the HTF are freezing (or salt solidification) and corrosion.
Thus, conventional coating techniques have been developed to address the hydrogen diffusion issue. For example, native thermal oxide on stainless steel tubes and aluminum oxide are used to mitigate the hydrogen permeation problem. In addition to addressing the hydrogen diffusion issue, the conventional steel central tube 12 typically includes a number of coatings on the outside surface thereof to achieve several functions, namely: (i) to promote the absorption of sun radiation; (ii) to promote reflection of sun radiation in the infrared spectrum; (iii) to promote low emissivity; and (iv) to act as a hydrogen barrier. For example, a typical coating configuration includes (from innermost to outermost layer): a hydrogen barrier of Al2O3 of about 25 nm thickness deposited on a native oxide; an infrared mirror of Mo of about 150 nm; a visible absorption layer of cermet (ceramic metal, Mo—Al2O3) of about 70-100 nm; and an anti-reflective layer of SiO2 or Al2O3 of about ¼ wavelength thickness.
In addition to coating the steel tube 12, getter material is introduced into the space 16 to remove hydrogen. Getter material, which combines or reacts with the hydrogen gas, assists in maintaining the vacuum. When the capacity of the getter material is saturated, the pressure again rises in the space 16 until the partial pressure of the free hydrogen in the space 16 reaches equilibrium with the hydrogen dissolved in the HTF. The equilibration pressure of the hydrogen in the space 16 amounts to between 0.3 mbar and 3 mbar in known absorber tubes.
There are a number of problems with the conventional HCE design, including: the relatively high complexity (and associated parts and assembly cost) of the bellows mechanism, heat loss through the bellows mechanism (which may be as high as 10%), the aforementioned thermal expansion mismatches and associated temperature instabilities, the hydrogen permeability problems discussed above, corrosion problems when molten salt HTFs are employed, complex and costly coating techniques, relatively high weight, etc.
Thus, there are needs in the art for new HCE mechanisms that reduce or eliminate one or more of the above problems.